P&S researchers have developed a simple blood test that
can identify postoperative thyroid cancer patients at risk for
metastatic disease. The test was described in the December 1996
issue of the journal Surgery.

The test involves a technique called reverse
transcriptase-polymerase chain reaction (RT-PCR), which allows
scientists to isolate minute amounts of genetic material (messenger
RNA), transcribe the material into DNA, and then clone enough copies
that can be detected using standardized tests. In this application,
RT-PCR is used to isolate and clone the mRNA for thyroglobulin, a
protein secreted only by thyroid cells. Normally, thyroid cells are
found only in the thyroid gland, explains Dr. Beth Ann Ditkoff,
assistant professor of surgery in the Thyroid Center and co-leader
of the research team. "So if thyroglobulin RNA is found in the
peripheral blood, this is an indication that thyroid cells are being
shed from metastatic tumors," she says. The thyroid is a small,
butterfly-shaped gland that lies over the trachea and below the
larynx. The gland produces thyroid hormone, which affects the rate
of virtually all metabolic processes in the body. Millions of
Americans have thyroid problems, such as hypothyroidism, in which
the gland produces too little hormone, or hyperthyroidism (such as
Grave's disease), in which it produces too much.

Cancer also can strike the thyroid. About 14,000 new cases
of thyroid cancer are diagnosed each year in the United States, says
Dr. Ditkoff. Most can be treated successfully with surgery, but
patients need to be closely monitored because metastases can occur
many years after the operation. "Right now, there's no perfect way
to monitor thyroid cancer patients postoperatively," says Dr. Paul
Lo Gerfo, co-director of the Thyroid Center and co-leader of the
research team.

One existing method is radioiodine scanning, which is
expensive and time-consuming and requires the patient to stop taking
thyroid replacement hormone for at least two weeks. "It's often very
difficult for patients to endure this hypothyroid state, which causes
extreme lethargy and sometimes depression," says Dr. Lo Gerfo. The
only other method, serum thyroglobulin measurement, is much simpler,
but it too has major drawbacks. False positive results are common,
since thyroglobulin is sometimes found in the blood of disease-free
individuals, in patients with benign thyroid disease, and, especially,
in thyroid cancer patients who have had only part of their glands
removed. Serum thyroglobulin testing is also plagued by false
negative results. "Not every tumor makes a significant amount of
thyroglobulin," says Dr. Lo Gerfo. "In addition, the body produces
antibodies to thyroglobulin, which can make the test difficult to
interpret."

The RT-PCR test, in contrast, is simple, inexpensive, and
accurate. Since the test detects the presence of thyroglobulin
mRNA--instead of thyroglobulin protein--it provides a clear
indication that thyroid cells are present in the blood and that the
patient is at risk for metastatic disease. The RT-PCR test is most
accurate when conducted more than three weeks after surgery because
surgery can temporarily release both benign and malignant thyroid
cells into the bloodstream, producing a false positive result. After
three weeks, and possibly sooner, however, these wayward cells
disappear from the blood. Says Dr. Ditkoff: "We're in the process
of taking serial blood samples from patients to see when these cells
clear after the operation and to see when positives are indeed true
positives. It looks right now that about three weeks afterward, the
test is accurate."

Dr. Ditkoff and her colleagues evaluated the accuracy
of the RT-PCR test in a clinical trial involving 100 patients,
including 78 with localized thyroid cancer, nine with metastatic
thyroid cancer, six with benign thyroid disease, and a control
group of seven disease-free individuals. Investigators conducting
the RT-PCR tests were blinded to the patients' actual clinical
status. All patients with benign disease and all controls tested
negative for circulating thyroglobulin mRNA. All patients with
metastatic disease tested positive. And significantly, seven
of the 78 patients with localized cancer also tested positive.
"Five of these seven patients had a past history of metastatic
thyroid cancer that had been successfully treated with surgery,"
says Dr. Ditkoff. "We know that patients like these are at risk
for a recurrence of metastatic disease, so their positive tests
suggest that they have circulating thyroid cancer cells that could
take root and form tumors. This is a subset of patients who are at
risk for metastatic disease."

Further research on the RT-PCR test is now in progress,
including tests of accuracy both preoperatively and postoperatively,
studies about whether the test can be used in place of radioiodine
scanning for long-term follow-up, and a study of additional markers
for metastatic disease. The investigators are also exploring a
possible link between the quantity of circulating thyroid cells and
a patient's prognosis. "First, we would have to develop a way to
quantitate how many thyroid cells are in the peripheral blood," Dr.
Ditkoff says. "Then we can examine whether that number influences
staging and prognosis." The other members of the research team are
Drs. Michael Marvin, John Chabot, and Carl Feind.